Photodiodes are light receiving elements that convert optical signals into electrical signals. Photodiodes are widely available in optical communication and optical pick-up devices such as CD-ROMs and DVDs. When light is incident onto a photodiode, electron-hole pairs are produced in a depletion region. The electron-hole pairs are transferred via an externally connected gate or an interconnect line. The current within a photodiode is varied substantially in accordance with an optical generation rate of carriers, and operates to convert the temporally varied optical signal to an electrical signal.
An image sensor with a photodiode (e.g., a CMOS image sensor (CIS)), includes a photo-sensing region and a periphery region. In the photo-sensing region, a plurality of unit pixels is arranged. The unit pixel has a single photodiode and a plurality of gates. The plurality of gates transmit the current generated from the photodiode to the periphery region. Then, the periphery region converts the current transferred from the photo-sensing region to digital signals using a plurality of circuits, thereby obtaining image information.
A device isolating layer is formed to separate a predetermined portion of the photodiode and circuits of the periphery circuit region from surrounding regions. The circuits of the periphery region are fabricated via a CMOS process, causing no problem in the device isolation. However, a device isolating layer around the photodiode may incite a dark current due to crystalline defects at an interfacial surface.
FIG. 1 is a sectional view illustrating a periphery of a photodiode including a device isolating layer in a conventional image sensor. Referring to FIG. 1, the image sensor includes a photodiode 20 formed on a semiconductor substrate 10, and separated by a device isolating layer 16. The semiconductor substrate 10 includes a first p-well 12 doped at a high density and a second p-well 14 doped at a relatively low density. The photodiode 20 generally has a p-type photodiode region 21 and an n-type photodiode region 22. A transfer gate 26, which is formed on a gate insulating layer 24, transfers optical charge generated from the photodiode 20 to a floating diffusion node 28.
The device isolating layer 16 is an insulating layer that inhibits overflow or signal interference between adjacent photodiodes 20, using STI or LOCOS. However, when forming the device isolating layer 16, crystalline defects may occur in an interfacial surface between the device isolating layer 16 and the first well 14. The crystalline defect functions as a noise component of a unit pixel and may increase dark current, which may damage picture quality of the image sensor.